Adding jsdoc for the optional promise behavior.
Adding jsdoc for the optional promise behavior.

RELNOTES: Added jsdoc.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162650124

lucidsoftware/closure-library,platinumpixs/google-closure-library,teppeis/closure-library,google/closure-library,platinumpixs/google-closure-library,teppeis/closure-library,teppeis/closure-library,google/closure-library,google/closure-library,google/closure-library,teppeis/closure-library,lucidsoftware/closure-library,platinumpixs/google-closure-library,platinumpixs/google-closure-library,lucidsoftware/closure-library,lucidsoftware/closure-library,google/closure-library